Spend an hour with Bubba Sparks and you’re a goner. You’ll become a vaulter. In the case of author John H. Clark III, all it took was a Facebook query to Bubba, his older classmate from Houston’s Scarborough High School. “Do you think I could take up pole vaulting for the first time in my life at 60 years old?” Of course, Bubba said: “Let’s go!” John’s journey from “couch potato” to National Senior Games entrant is movingly told in his latest book — “Finally Fit: It’s Never Too Late to Achieve a Dream.” Published six days after Valentine’s Day,…